The present invention relates in general to electromagnetic machines, and more specifically to the structure and the method of manufacturing rotors in electromagnetic machines.
Induction machines with rotors incorporating compound short-circuit loops with open loop portions have been shown to have certain performance advantages over traditionalxe2x80x9csquirrel-cagexe2x80x9d induction machines. U.S. Pat. No. 4,761,602, issued on Aug. 2, 1988 to Leibovich and entitled xe2x80x9cCompound Short-Circuit Induction Machine and Method of Its Control,xe2x80x9d and which is incorporated herein by reference in its entirety, demonstrates these performance advantages and also discloses one possible construction of such a compound loop in relative position to a rotor.
The enhancements in performance of the compound short-circuit induction machine come, however, at a price of increased complexity in manufacturing. The rotors of such machines must be manufactured such that the windings of each three-dimensional compound loop are positioned within slots located at the peripheral portion of the rotor. In an N-phase induction machine, there are typically N compound loops that must overlap in three-dimensional space without coming into contact or xe2x80x9cshort circuitingxe2x80x9d with one another, with every winding of each compound loop positioned within one of the said slots. Winding and connecting the windings in this configuration present significant challenges in manufacturing.
By contrast, ease of manufacturing is well-noted as a primary advantage of the traditional xe2x80x9csquirrel-cagexe2x80x9d induction machine. In these machines, die-cast aluminum bar members are typically formed in the laminations, thereby replacing the need for windings on the rotor. In this configuration, end rings are attached to the bar members by welding, brazing, or die-casting to form the xe2x80x9csquirrel-cagexe2x80x9d structure. This simple structure and method of construction of the squirrel-cage design has been utilized by numerous rotating machines. For example, U.S. Pat. No. 5,028,804, issued on Jul. 2, 1991 to Lauw, et al. and entitled xe2x80x9cBrushless Doubly-Fed Generator Control System,xe2x80x9d and which is incorporated herein by reference in its entirety, describes the formation of U-shaped coils by removing one of the end rings of a squirrel-cage structure and connecting appropriate bar members. U.S. Pat. No. 4,692,647, issued on Sept. 8, 1987 to Oh, et al. and entitled xe2x80x9cRotor Construction in an Induction Motor for Eliminating Rotor Reaction,xe2x80x9d and incorporated herein by reference in its entirety, describes the formation of multiple, two-dimensional ring-shaped coils by die-casting the coils within the rotor to form a single body.
Adaptation of the simple structure and method of construction of the die-cast squirrel-cage design to manufacture a rotor containing multiple compound short-circuit loops would make manufacturing such a rotor easier, but the complexity of the compound loop structure has heretofore prevented such an adaptation. Thus, there is a need in the art for a machine that combines the performance advantages of the compound short-circuit induction machine with the ease of manufacture associated with the die-cast squirrel-cage induction machine.
The present invention in a broad aspect addresses the problems and shortcomings mentioned above. Specifically, an embodiment of the invention involves the use of endshield laminations to be placed on the top and bottom of a lamination stack of a rotor. The endshields have holes therein which mask the ends of the bar members within the lamination stack. The unmasked portions of the bar members are connected through the holes to connection members in such a fashion to make multiple compound short-circuit loops. Because the endshields, like the laminations generally, are not conductive, the connection members are only in contact with the bar members through the holes in the endshield, which prevents the multiple loops from shorting to one another. The bar members and connector members are preferably die-cast in and around the rotor laminations and the endshields.